diff --git a/config.json b/config.json
index 2c61446a..773d064c 100644
--- a/config.json
+++ b/config.json
@@ -655,6 +655,17 @@
           "transforming"
         ]
       },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "c3fbce6a-61ce-439c-8c67-fad93561326f",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4,
+        "topics": [
+          "dates"
+        ]
+      },
       {
         "slug": "binary-search",
         "name": "Binary Search",
diff --git a/exercises/practice/baffling-birthdays/.docs/instructions.md b/exercises/practice/baffling-birthdays/.docs/instructions.md
new file mode 100644
index 00000000..a01ec867
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/.docs/instructions.md
@@ -0,0 +1,23 @@
+# Instructions
+
+Your task is to estimate the birthday paradox's probabilities.
+
+To do this, you need to:
+
+- Generate random birthdates.
+- Check if a collection of randomly generated birthdates contains at least two with the same birthday.
+- Estimate the probability that at least two people in a group share the same birthday for different group sizes.
+
+~~~~exercism/note
+A birthdate includes the full date of birth (year, month, and day), whereas a birthday refers only to the month and day, which repeat each year.
+Two birthdates with the same month and day correspond to the same birthday.
+~~~~
+
+~~~~exercism/caution
+The birthday paradox assumes that:
+
+- There are 365 possible birthdays (no leap years).
+- Each birthday is equally likely (uniform distribution).
+
+Your implementation must follow these assumptions.
+~~~~
diff --git a/exercises/practice/baffling-birthdays/.docs/introduction.md b/exercises/practice/baffling-birthdays/.docs/introduction.md
new file mode 100644
index 00000000..97dabd1e
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/.docs/introduction.md
@@ -0,0 +1,25 @@
+# Introduction
+
+Fresh out of college, you're throwing a huge party to celebrate with friends and family.
+Over 70 people have shown up, including your mildly eccentric Uncle Ted.
+
+In one of his usual antics, he bets you £100 that at least two people in the room share the same birthday.
+That sounds ridiculous — there are many more possible birthdays than there are guests, so you confidently accept.
+
+To your astonishment, after collecting the birthdays of just 32 guests, you've already found two guests that share the same birthday.
+Accepting your loss, you hand Uncle Ted his £100, but something feels off.
+
+The next day, curiosity gets the better of you.
+A quick web search leads you to the [birthday paradox][birthday-problem], which reveals that with just 23 people, the probability of a shared birthday exceeds 50%.
+
+Ah. So _that's_ why Uncle Ted was so confident.
+
+Determined to turn the tables, you start looking up other paradoxes; next time, _you'll_ be the one making the bets.
+
+~~~~exercism/note
+The birthday paradox is a [veridical paradox][veridical-paradox]: even though it feels wrong, it is actually true.
+
+[veridical-paradox]: https://en.wikipedia.org/wiki/Paradox#Quine's_classification
+~~~~
+
+[birthday-problem]: https://en.wikipedia.org/wiki/Birthday_problem
diff --git a/exercises/practice/baffling-birthdays/.meta/config.json b/exercises/practice/baffling-birthdays/.meta/config.json
new file mode 100644
index 00000000..d9197e1e
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "resu-xuniL"
+  ],
+  "files": {
+    "solution": [
+      "baffling_birthdays.sh"
+    ],
+    "test": [
+      "baffling_birthdays.bats"
+    ],
+    "example": [
+      ".meta/example.sh"
+    ]
+  },
+  "blurb": "Estimate the birthday paradox's probabilities.",
+  "source": "Erik Schierboom",
+  "source_url": "https://github.com/exercism/problem-specifications/pull/2539"
+}
diff --git a/exercises/practice/baffling-birthdays/.meta/example.sh b/exercises/practice/baffling-birthdays/.meta/example.sh
new file mode 100644
index 00000000..b2111103
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/.meta/example.sh
@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+
+current_year=$(date -d "now" +%Y)
+
+main() {
+    local function=$1; shift
+
+    "$function" "$@"
+}
+
+shared_birthday () {
+    local -a birthdates=( "$@" )
+    local -A birthdays
+
+    for birthdate in "${birthdates[@]}"; do
+        birthday=${birthdate: -5}
+
+        (( birthdays[$birthday]++ ))
+    done
+
+    if (( ${#birthdays[@]} == ${#birthdates[@]} )); then
+        echo "false"
+    else
+        echo "true"
+    fi
+}
+
+random_int() {
+    echo $(( RANDOM % $1 ))
+}
+
+random_birthdates () {
+    local -i number=$1
+    local -i random_year random_month random_day
+    local -a birthdates
+
+    for (( i = 0; i < number; i++ )); do
+        random_year=$(( 1900 + $(random_int $(( current_year - 1900 + 1 ))) ))
+        random_month=$(( 1 + $(random_int 12) ))
+
+        if (( random_year % 400 == 0 || (random_year % 4 == 0 && random_year % 100 != 0) )); then
+            (( random_year ++ ))
+        fi
+
+        case "$random_month" in
+                   2) random_day=$(( 1 + $(random_int 28) )) ;;
+            4|6|9|11) random_day=$(( 1 + $(random_int 30) )) ;;
+                   *) random_day=$(( 1 + $(random_int 31) )) ;;
+        esac
+
+        birthdates+=( "$(printf '%d-%02d-%02d' "$random_year" "$random_month" "$random_day")" )
+    done
+
+    echo "${birthdates[@]}"
+}
+
+estimated_probability_of_share_birthday () {
+    local -i group_size=$1
+    # the more you increase the 'runs' value,
+    # the more the result is approching the expected one
+    # but the more you increase the computing time!
+    local -i runs=2000
+    local -i count=0
+
+    for (( i = 0; i <= runs; i++ )); do
+        read -ra birthdates < <(random_birthdates "$group_size")
+        if [[ "$(shared_birthday "${birthdates[@]}")" == "true" ]]; then
+            (( count++ ))
+        fi
+    done
+
+    echo $(( count * 100 / runs ))
+}
+
+main "$@"
diff --git a/exercises/practice/baffling-birthdays/.meta/tests.toml b/exercises/practice/baffling-birthdays/.meta/tests.toml
new file mode 100644
index 00000000..c76afb46
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/.meta/tests.toml
@@ -0,0 +1,61 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[716dcc2b-8fe4-4fc9-8c48-cbe70d8e6b67]
+description = "shared birthday -> one birthdate"
+
+[f7b3eb26-bcfc-4c1e-a2de-af07afc33f45]
+description = "shared birthday -> two birthdates with same year, month, and day"
+
+[7193409a-6e16-4bcb-b4cc-9ffe55f79b25]
+description = "shared birthday -> two birthdates with same year and month, but different day"
+
+[d04db648-121b-4b72-93e8-d7d2dced4495]
+description = "shared birthday -> two birthdates with same month and day, but different year"
+
+[3c8bd0f0-14c6-4d4c-975a-4c636bfdc233]
+description = "shared birthday -> two birthdates with same year, but different month and day"
+
+[df5daba6-0879-4480-883c-e855c99cdaa3]
+description = "shared birthday -> two birthdates with different year, month, and day"
+
+[0c17b220-cbb9-4bd7-872f-373044c7b406]
+description = "shared birthday -> multiple birthdates without shared birthday"
+
+[966d6b0b-5c0a-4b8c-bc2d-64939ada49f8]
+description = "shared birthday -> multiple birthdates with one shared birthday"
+
+[b7937d28-403b-4500-acce-4d9fe3a9620d]
+description = "shared birthday -> multiple birthdates with more than one shared birthday"
+
+[70b38cea-d234-4697-b146-7d130cd4ee12]
+description = "random birthdates -> generate requested number of birthdates"
+
+[d9d5b7d3-5fea-4752-b9c1-3fcd176d1b03]
+description = "random birthdates -> years are not leap years"
+
+[d1074327-f68c-4c8a-b0ff-e3730d0f0521]
+description = "random birthdates -> months are random"
+
+[7df706b3-c3f5-471d-9563-23a4d0577940]
+description = "random birthdates -> days are random"
+
+[89a462a4-4265-4912-9506-fb027913f221]
+description = "estimated probability of at least one shared birthday -> for one person"
+
+[ec31c787-0ebb-4548-970c-5dcb4eadfb5f]
+description = "estimated probability of at least one shared birthday -> among ten people"
+
+[b548afac-a451-46a3-9bb0-cb1f60c48e2f]
+description = "estimated probability of at least one shared birthday -> among twenty-three people"
+
+[e43e6b9d-d77b-4f6c-a960-0fc0129a0bc5]
+description = "estimated probability of at least one shared birthday -> among seventy people"
diff --git a/exercises/practice/baffling-birthdays/baffling_birthdays.bats b/exercises/practice/baffling-birthdays/baffling_birthdays.bats
new file mode 100644
index 00000000..3a1e1b9c
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/baffling_birthdays.bats
@@ -0,0 +1,178 @@
+#!/usr/bin/env bats
+load bats-extra
+
+@test "shared birthday -> one birthdate" {
+    #[[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("2000-01-01")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "false"
+}
+
+@test "shared birthday -> two birthdates with same year, month, and day" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("2000-01-01" "2000-01-01")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "true"
+}
+
+@test "shared birthday -> two birthdates with same year and month, but different day" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("2012-05-09" "2012-05-17")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "false"
+}
+
+@test "shared birthday -> two birthdates with same month and day, but different year" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("1999-10-23" "1988-10-23")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "true"
+}
+
+@test "shared birthday -> two birthdates with same year, but different month and day" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("2007-12-19" "2007-04-27")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "false"
+}
+
+@test "shared birthday -> two birthdates with different year, month, and day" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=("1997-08-04" "1963-11-23")
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "false"
+}
+
+@test "shared birthday -> multiple birthdates without shared birthday" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=(
+        "1966-07-29"
+        "1977-02-12"
+        "2001-12-25"
+        "1980-11-10"
+    )
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "false"
+}
+
+@test "shared birthday -> multiple birthdates with one shared birthday" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=(
+        "1966-07-29"
+        "1977-02-12"
+        "2001-07-29"
+        "1980-11-10"
+    )
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "true"
+}
+
+@test "shared birthday -> multiple birthdates with more than one shared birthday" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    birthdates=(
+        "1966-07-29"
+        "1977-02-12"
+        "2001-12-25"
+        "1980-07-29"
+        "2019-02-12"
+    )
+    run bash baffling_birthdays.sh shared_birthday "${birthdates[@]}"
+    assert_success
+    assert_output "true"
+}
+
+@test "random birthdates -> generate requested number of birthdates" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    generate=500
+    run bash baffling_birthdays.sh random_birthdates "$generate"
+    assert_success
+    num_output_dates=$( wc -w <<< "$output" )
+    assert_equal "$num_output_dates" "$generate"
+}
+
+@test "random birthdates -> years are not leap years" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    generate=500
+    seen_leap=0
+    run bash baffling_birthdays.sh random_birthdates "$generate"
+    assert_success
+    read -ra output_dates <<< "$output"
+    for date in "${output_dates[@]}"; do
+        year=${date:0:4}
+        if (( year % 400 == 0 || (year % 4 == 0 && year % 100 != 0) )); then
+            (( ++seen_leap ))
+        fi
+    done
+    assert_equal "${seen_leap}" 0
+}
+
+@test "random birthdates -> months are random" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    declare -A seen_month
+    generate=500
+    run bash baffling_birthdays.sh random_birthdates "$generate"
+    assert_success
+    read -ra output_dates <<< "$output"
+    for date in "${output_dates[@]}"; do
+        month=${date:5:2}
+
+        seen_month[$month]=true
+    done
+    assert_equal "${#seen_month[@]}" 12
+}
+
+@test "random birthdates -> days are random" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    declare -A seen_day
+    generate=500
+    run bash baffling_birthdays.sh random_birthdates "$generate"
+    assert_success
+    read -ra output_dates <<< "$output"
+    for date in "${output_dates[@]}"; do
+        day=${date:8}
+
+        seen_day[$day]=true
+    done
+    assert_equal "${#seen_day[@]}" 31
+}
+
+# "The expected probability values should be compared using some tolerance to allow for small deviations."
+@test "estimated probability of at least one shared birthday -> for one person" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    # The expected probability is 0.0
+    run bash baffling_birthdays.sh estimated_probability_of_share_birthday 1
+    assert_success
+    assert_equal "$output" 0
+}
+
+@test "estimated probability of at least one shared birthday -> among ten people" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    # The expected probability is 11.694818
+    run bash baffling_birthdays.sh estimated_probability_of_share_birthday 10
+    assert_success
+    assert_between 10 13
+}
+
+@test "estimated probability of at least one shared birthday -> among twenty-three people" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    # The expected probability is 50.729723
+    run bash baffling_birthdays.sh estimated_probability_of_share_birthday 23
+    assert_success
+    assert_between 48 53
+}
+
+@test "estimated probability of at least one shared birthday -> among seventy people" {
+    [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+    # The expected probability is 99.915958
+    run bash baffling_birthdays.sh estimated_probability_of_share_birthday 70
+    assert_success
+    assert_between 98 100
+}
diff --git a/exercises/practice/baffling-birthdays/baffling_birthdays.sh b/exercises/practice/baffling-birthdays/baffling_birthdays.sh
new file mode 100644
index 00000000..960869f3
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/baffling_birthdays.sh
@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# The following comments should help you get started:
+# - Bash is flexible. You may use functions or write a "raw" script.
+#
+# - Complex code can be made easier to read by breaking it up
+#   into functions, however this is sometimes overkill in bash.
+#
+# - You can find links about good style and other resources
+#   for Bash in './README.md'. It came with this exercise.
+#
+#   Example:
+#   # other functions here
+#   # ...
+#   # ...
+#
+#   main () {
+#     # your main function code here
+#   }
+#
+#   # call main with all of the positional arguments
+#   main "$@"
+#
+# *** PLEASE REMOVE THESE COMMENTS BEFORE SUBMITTING YOUR SOLUTION ***
diff --git a/exercises/practice/baffling-birthdays/bats-extra.bash b/exercises/practice/baffling-birthdays/bats-extra.bash
new file mode 100644
index 00000000..faf3899e
--- /dev/null
+++ b/exercises/practice/baffling-birthdays/bats-extra.bash
@@ -0,0 +1,653 @@
+# This is the source code for bats-support and bats-assert, concatenated
+# * https://github.com/bats-core/bats-support
+# * https://github.com/bats-core/bats-assert
+#
+# Comments have been removed to save space. See the git repos for full source code.
+
+############################################################
+#
+# bats-support - Supporting library for Bats test helpers
+#
+# Written in 2016 by Zoltan Tombol <zoltan dot tombol at gmail dot com>
+#
+# To the extent possible under law, the author(s) have dedicated all
+# copyright and related and neighboring rights to this software to the
+# public domain worldwide. This software is distributed without any
+# warranty.
+#
+# You should have received a copy of the CC0 Public Domain Dedication
+# along with this software. If not, see
+# <http://creativecommons.org/publicdomain/zero/1.0/>.
+#
+
+fail() {
+  (( $# == 0 )) && batslib_err || batslib_err "$@"
+  return 1
+}
+
+batslib_is_caller() {
+  local -i is_mode_direct=1
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+      -i|--indirect) is_mode_direct=0; shift ;;
+      --) shift; break ;;
+      *) break ;;
+    esac
+  done
+
+  # Arguments.
+  local -r func="$1"
+
+  # Check call stack.
+  if (( is_mode_direct )); then
+    [[ $func == "${FUNCNAME[2]}" ]] && return 0
+  else
+    local -i depth
+    for (( depth=2; depth<${#FUNCNAME[@]}; ++depth )); do
+      [[ $func == "${FUNCNAME[$depth]}" ]] && return 0
+    done
+  fi
+
+  return 1
+}
+
+batslib_err() {
+  { if (( $# > 0 )); then
+      echo "$@"
+    else
+      cat -
+    fi
+  } >&2
+}
+
+batslib_count_lines() {
+  local -i n_lines=0
+  local line
+  while IFS='' read -r line || [[ -n $line ]]; do
+    (( ++n_lines ))
+  done < <(printf '%s' "$1")
+  echo "$n_lines"
+}
+
+batslib_is_single_line() {
+  for string in "$@"; do
+    (( $(batslib_count_lines "$string") > 1 )) && return 1
+  done
+  return 0
+}
+
+batslib_get_max_single_line_key_width() {
+  local -i max_len=-1
+  while (( $# != 0 )); do
+    local -i key_len="${#1}"
+    batslib_is_single_line "$2" && (( key_len > max_len )) && max_len="$key_len"
+    shift 2
+  done
+  echo "$max_len"
+}
+
+batslib_print_kv_single() {
+  local -ir col_width="$1"; shift
+  while (( $# != 0 )); do
+    printf '%-*s : %s\n' "$col_width" "$1" "$2"
+    shift 2
+  done
+}
+
+batslib_print_kv_multi() {
+  while (( $# != 0 )); do
+    printf '%s (%d lines):\n' "$1" "$( batslib_count_lines "$2" )"
+    printf '%s\n' "$2"
+    shift 2
+  done
+}
+
+batslib_print_kv_single_or_multi() {
+  local -ir width="$1"; shift
+  local -a pairs=( "$@" )
+
+  local -a values=()
+  local -i i
+  for (( i=1; i < ${#pairs[@]}; i+=2 )); do
+    values+=( "${pairs[$i]}" )
+  done
+
+  if batslib_is_single_line "${values[@]}"; then
+    batslib_print_kv_single "$width" "${pairs[@]}"
+  else
+    local -i i
+    for (( i=1; i < ${#pairs[@]}; i+=2 )); do
+      pairs[$i]="$( batslib_prefix < <(printf '%s' "${pairs[$i]}") )"
+    done
+    batslib_print_kv_multi "${pairs[@]}"
+  fi
+}
+
+batslib_prefix() {
+  local -r prefix="${1:-  }"
+  local line
+  while IFS='' read -r line || [[ -n $line ]]; do
+    printf '%s%s\n' "$prefix" "$line"
+  done
+}
+
+batslib_mark() {
+  local -r symbol="$1"; shift
+  # Sort line numbers.
+  set -- $( sort -nu <<< "$( printf '%d\n' "$@" )" )
+
+  local line
+  local -i idx=0
+  while IFS='' read -r line || [[ -n $line ]]; do
+    if (( ${1:--1} == idx )); then
+      printf '%s\n' "${symbol}${line:${#symbol}}"
+      shift
+    else
+      printf '%s\n' "$line"
+    fi
+    (( ++idx ))
+  done
+}
+
+batslib_decorate() {
+  echo
+  echo "-- $1 --"
+  cat -
+  echo '--'
+  echo
+}
+
+############################################################
+
+assert() {
+  if ! "$@"; then
+    batslib_print_kv_single 10 'expression' "$*" \
+    | batslib_decorate 'assertion failed' \
+    | fail
+  fi
+}
+
+assert_equal() {
+  if [[ $1 != "$2" ]]; then
+    batslib_print_kv_single_or_multi 8 \
+    'expected' "$2" \
+    'actual'   "$1" \
+    | batslib_decorate 'values do not equal' \
+    | fail
+  fi
+}
+
+assert_failure() {
+  : "${output?}"
+  : "${status?}"
+
+  (( $# > 0 )) && local -r expected="$1"
+  if (( status == 0 )); then
+    batslib_print_kv_single_or_multi 6 'output' "$output" \
+    | batslib_decorate 'command succeeded, but it was expected to fail' \
+    | fail
+  elif (( $# > 0 )) && (( status != expected )); then
+    { local -ir width=8
+      batslib_print_kv_single "$width" \
+      'expected' "$expected" \
+      'actual'   "$status"
+      batslib_print_kv_single_or_multi "$width" \
+      'output' "$output"
+    } \
+    | batslib_decorate 'command failed as expected, but status differs' \
+    | fail
+  fi
+}
+
+assert_line() {
+  local -i is_match_line=0
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  : "${lines?}"
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+    -n|--index)
+      if (( $# < 2 )) || ! [[ $2 =~ ^([0-9]|[1-9][0-9]+)$ ]]; then
+        echo "\`--index' requires an integer argument: \`$2'" \
+        | batslib_decorate 'ERROR: assert_line' \
+        | fail
+        return $?
+      fi
+      is_match_line=1
+      local -ri idx="$2"
+      shift 2
+      ;;
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: assert_line' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local -r expected="$1"
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $expected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$expected'" \
+    | batslib_decorate 'ERROR: assert_line' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_match_line )); then
+    # Specific line.
+    if (( is_mode_regexp )); then
+      if ! [[ ${lines[$idx]} =~ $expected ]]; then
+        batslib_print_kv_single 6 \
+        'index'  "$idx" \
+        'regexp' "$expected" \
+        'line'   "${lines[$idx]}" \
+        | batslib_decorate 'regular expression does not match line' \
+        | fail
+      fi
+    elif (( is_mode_partial )); then
+      if [[ ${lines[$idx]} != *"$expected"* ]]; then
+        batslib_print_kv_single 9 \
+        'index'     "$idx" \
+        'substring' "$expected" \
+        'line'      "${lines[$idx]}" \
+        | batslib_decorate 'line does not contain substring' \
+        | fail
+      fi
+    else
+      if [[ ${lines[$idx]} != "$expected" ]]; then
+        batslib_print_kv_single 8 \
+        'index'    "$idx" \
+        'expected' "$expected" \
+        'actual'   "${lines[$idx]}" \
+        | batslib_decorate 'line differs' \
+        | fail
+      fi
+    fi
+  else
+    # Contained in output.
+    if (( is_mode_regexp )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} =~ $expected ]] && return 0
+      done
+      { local -ar single=( 'regexp' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'no output line matches regular expression' \
+      | fail
+    elif (( is_mode_partial )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} == *"$expected"* ]] && return 0
+      done
+      { local -ar single=( 'substring' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'no output line contains substring' \
+      | fail
+    else
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} == "$expected" ]] && return 0
+      done
+      { local -ar single=( 'line' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'output does not contain line' \
+      | fail
+    fi
+  fi
+}
+
+assert_output() {
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  local -i is_mode_nonempty=0
+  local -i use_stdin=0
+  : "${output?}"
+
+  # Handle options.
+  if (( $# == 0 )); then
+    is_mode_nonempty=1
+  fi
+
+  while (( $# > 0 )); do
+    case "$1" in
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    -|--stdin) use_stdin=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: assert_output' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local expected
+  if (( use_stdin )); then
+    expected="$(cat -)"
+  else
+    expected="${1-}"
+  fi
+
+  # Matching.
+  if (( is_mode_nonempty )); then
+    if [ -z "$output" ]; then
+      echo 'expected non-empty output, but output was empty' \
+      | batslib_decorate 'no output' \
+      | fail
+    fi
+  elif (( is_mode_regexp )); then
+    if [[ '' =~ $expected ]] || (( $? == 2 )); then
+      echo "Invalid extended regular expression: \`$expected'" \
+      | batslib_decorate 'ERROR: assert_output' \
+      | fail
+    elif ! [[ $output =~ $expected ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'regexp'  "$expected" \
+      'output'  "$output" \
+      | batslib_decorate 'regular expression does not match output' \
+      | fail
+    fi
+  elif (( is_mode_partial )); then
+    if [[ $output != *"$expected"* ]]; then
+      batslib_print_kv_single_or_multi 9 \
+      'substring' "$expected" \
+      'output'    "$output" \
+      | batslib_decorate 'output does not contain substring' \
+      | fail
+    fi
+  else
+    if [[ $output != "$expected" ]]; then
+      batslib_print_kv_single_or_multi 8 \
+      'expected' "$expected" \
+      'actual'   "$output" \
+      | batslib_decorate 'output differs' \
+      | fail
+    fi
+  fi
+}
+
+assert_success() {
+  : "${output?}"
+  : "${status?}"
+
+  if (( status != 0 )); then
+    { local -ir width=6
+      batslib_print_kv_single "$width" 'status' "$status"
+      batslib_print_kv_single_or_multi "$width" 'output' "$output"
+    } \
+    | batslib_decorate 'command failed' \
+    | fail
+  fi
+}
+
+refute() {
+  if "$@"; then
+    batslib_print_kv_single 10 'expression' "$*" \
+    | batslib_decorate 'assertion succeeded, but it was expected to fail' \
+    | fail
+  fi
+}
+
+refute_line() {
+  local -i is_match_line=0
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  : "${lines?}"
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+    -n|--index)
+      if (( $# < 2 )) || ! [[ $2 =~ ^([0-9]|[1-9][0-9]+)$ ]]; then
+        echo "\`--index' requires an integer argument: \`$2'" \
+        | batslib_decorate 'ERROR: refute_line' \
+        | fail
+        return $?
+      fi
+      is_match_line=1
+      local -ri idx="$2"
+      shift 2
+      ;;
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: refute_line' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local -r unexpected="$1"
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $unexpected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$unexpected'" \
+    | batslib_decorate 'ERROR: refute_line' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_match_line )); then
+    # Specific line.
+    if (( is_mode_regexp )); then
+      if [[ ${lines[$idx]} =~ $unexpected ]]; then
+        batslib_print_kv_single 6 \
+        'index'  "$idx" \
+        'regexp' "$unexpected" \
+        'line'   "${lines[$idx]}" \
+        | batslib_decorate 'regular expression should not match line' \
+        | fail
+      fi
+    elif (( is_mode_partial )); then
+      if [[ ${lines[$idx]} == *"$unexpected"* ]]; then
+        batslib_print_kv_single 9 \
+        'index'     "$idx" \
+        'substring' "$unexpected" \
+        'line'      "${lines[$idx]}" \
+        | batslib_decorate 'line should not contain substring' \
+        | fail
+      fi
+    else
+      if [[ ${lines[$idx]} == "$unexpected" ]]; then
+        batslib_print_kv_single 5 \
+        'index' "$idx" \
+        'line'  "${lines[$idx]}" \
+        | batslib_decorate 'line should differ' \
+        | fail
+      fi
+    fi
+  else
+    # Line contained in output.
+    if (( is_mode_regexp )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} =~ $unexpected ]]; then
+          { local -ar single=( 'regexp' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'no line should match the regular expression' \
+          | fail
+          return $?
+        fi
+      done
+    elif (( is_mode_partial )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} == *"$unexpected"* ]]; then
+          { local -ar single=( 'substring' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'no line should contain substring' \
+          | fail
+          return $?
+        fi
+      done
+    else
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} == "$unexpected" ]]; then
+          { local -ar single=( 'line' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'line should not be in output' \
+          | fail
+          return $?
+        fi
+      done
+    fi
+  fi
+}
+
+refute_output() {
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  local -i is_mode_empty=0
+  local -i use_stdin=0
+  : "${output?}"
+
+  # Handle options.
+  if (( $# == 0 )); then
+    is_mode_empty=1
+  fi
+
+  while (( $# > 0 )); do
+    case "$1" in
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    -|--stdin) use_stdin=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: refute_output' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local unexpected
+  if (( use_stdin )); then
+    unexpected="$(cat -)"
+  else
+    unexpected="${1-}"
+  fi
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $unexpected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$unexpected'" \
+    | batslib_decorate 'ERROR: refute_output' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_mode_empty )); then
+    if [ -n "$output" ]; then
+      batslib_print_kv_single_or_multi 6 \
+      'output' "$output" \
+      | batslib_decorate 'output non-empty, but expected no output' \
+      | fail
+    fi
+  elif (( is_mode_regexp )); then
+    if [[ $output =~ $unexpected ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'regexp' "$unexpected" \
+      'output' "$output" \
+      | batslib_decorate 'regular expression should not match output' \
+      | fail
+    fi
+  elif (( is_mode_partial )); then
+    if [[ $output == *"$unexpected"* ]]; then
+      batslib_print_kv_single_or_multi 9 \
+      'substring' "$unexpected" \
+      'output'    "$output" \
+      | batslib_decorate 'output should not contain substring' \
+      | fail
+    fi
+  else
+    if [[ $output == "$unexpected" ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'output' "$output" \
+      | batslib_decorate 'output equals, but it was expected to differ' \
+      | fail
+    fi
+  fi
+}
+
+assert_between() {
+  local -r start="${1}"
+  local -r end="${2}"
+
+  if (( start > end )); then
+    return 1
+  elif ! (( output >= "$1" && output <= "$2" )); then
+    batslib_print_kv_single_or_multi 6 \
+      'start'  "${start}" \
+      'end'    "${end}" \
+      'actual' "${output}" \
+    | batslib_decorate 'output falls outside the inclusive range defined by start and end' \
+    | fail
+  fi
+}